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Patented Jan. 30, 1923.

UNITE HERBERT W. GEPP, OF MELBOURNE, VICTORIA, AUSTRALIA, ASSIGNOR TO ELECTRO- LY'IIC ZINC COMPANY OF AUSTRALASIA PROPRIETARY LIMITED, MELBOURNE,

AUSTRALIA.

ROASTING OF ZINC SULPI-IIDE GEES PREPABATORY TO LEACHIIN'G.

No Drawing.

To all whom 2'2? may concern Be it known that I, HERBERT WILLIAM GEPP, general manager of Electrolytic Zinc Company of Australasia Proprietary Lim- 5 ited, residing at Collins House, 360366 Collins Street, Melbourne, in the State of Victoria, Commonwealth of Australia, a subject of the King of Great Britain, have invented certain new and useful Improvements in the Roasting of Zinc Sulphide Ores Preparatory to Leaching, of which the following is a specification.

This invention relates to certain improvements in the roasting of sulphide ores preparatory to leaching and refers more espe cially to the roasting of zinc blende preparatory to leaching with sulphuric acid or spent electrolyte for the formation. of zinc sulphate solution prior to electrolysis for the deposition of metallic zinc or spelter.

The invention is applicable to any process which involves the preparation of solutions of zinc sulphate such as the manufacture of lithopone and such like processes.

This invention relates to the treatment of concentrates, slimes, tailings and other metallurgical products as well as to crude ores and it will be understood that in this specification the term ores will include metalliferous products as well as the crude material. p The methods which have become standard for the roasting of zinc sulphide ores for the distillation process have been found to be more or less unsatisfactory when the roasted ore is to be leached with sulphuric acid.

In the roasting of ores preparatory to distillation it is sought to eliminate the sulphur 40 whether present as sulphide or sulphate.

On the other hand when the ores are to be leached with sulphuric acid the presence of sulphates is not a disadvantage; in fact it may even be an advantage in cases where a steady consumption of sulphuric acid has to be met in the subsequent leaching. This of course, is true more or less in the electrolytic process, and still more so inthe manufacture of lithopone Where practically the whole of the sulphuric acid appears in the final product as barium sulphate.

' In the electrolytic zinc process it is usually necessary to add a quantity of sulphuric acid Serial No. 321,949.

to the leaching solution in order to make up losses occasioned in the working of the process and to maintain the proper concentration of zinc in the solution.

In 'the roasting of zinc sulphide ores when it is desired to effect the more or less complete elimination of the sulphur and to obtain a dead roast or sweet roast (such as is the practice preparatory to the distillation process) it has been found necessary to malntain the operation at a high temperature and if the gases are to be used for the manufacture of sulphuric acid the admis sion of air must be carefully controlled.

In the roasting of zinc sulphide ores when it i desired to effect a sulphating roast and to obtain a maximum proportion of metallic sulphates it is necessary to maintain the temperature as low as possible to prevent the breaking up of the metallic sulphates formed (and in particular zinc sulphate) and further an excess of air is required to effect the oxidation to sulphates which renders the gases too dilute for the manufacture of sulphuric acid.

The object of this invention is to provide certain improvements in the roasting of zinc sulphide ores preparatory to leaching whereby a sulphating roast is efiected with the formation of a maximum quantity of metallic sulphates which are introduced into the leaching solution thereby reducing the amount of sulphuric acid which would otherwise be required to be added to the leaching solutions or avoiding such additions altogether.

I accomplish this object by conducting the roasting operation under such conditions that a maximum of metallic sulphates are formed. This is preferably effected by conducting the roasting operation in two stages. The first stage is carried out under such conditions as obtain with an ordinary roast that is to say with the maintenance of a relatively high temperature and with a carefully controlled supply of air to ensure a sufiicient concentration of SO for the manufacture of sulphuric acid, the sulphur contents being reduced to such an amount as may be necessary for the subsequent operation. The sec ond stage is effected at a relatively lower temperature and under such conditions of air supply and amount of available sulphur present as is calculated to give a maximum yield of metallic sulphates.

This two-stage roasting operation may be conducted in either of two ways that is e ther b conducting the first operaticn for a partia elimination of the sulphur, and the second operation for the formation of a maximum amcum of sulphates 'or alarm. tively by conducting the first operation for the more or less complete elimination of the sulphur and then adding green or unroasted sul hides and roastingunder such conditions but a maximum amount of sulphates is formed.

I havediscovered that the 'eficiency of the sulphate formation increases asthe sulphide sulphur contents decrease. For example in the roasting of an ore containing 28% sulphur at a temperature suitable for the formation of sulphates it has been found that with the'reduction of the sulphur down to 8% only the lead was sulphated but that Whenthe reduction was continued it was possible to securea 40% conversion to sulphate of the remaining 8 units of sulphur. It was further found thatif the reduction of the su phur in the first instance had been carrie down to 6% it was possible to ob tain conversion to sulphate of those remaining 6 units. It has been found that the conversation of the last unit of sulphide sulphur to sulphate is almost quantitative.

In proceedin as indicated inthe former alternative -(t at is to say when conducting the first 0 ration for a artial elimination of the su hur) the pre iminary roasti is conducted in the same manner ashoreto ore, being effected in a suitable furnace at a relatively high temperature with regulation of air supply so that the gases are suitable for the formation of sulphuric acid.

In this preliminary roasting operation'the sulphur contents are reduced to approximatel from 6 to 10%. In the second 0 eration t e roastin is carried forward with a view to convertmg a maximum amount of sulphur to sulphate sulphur which is attained by definite control over both tem erature and draught and with the ad ition where necessary at certain stages of further sulphur such as by adding green or unroasted sulphides. The temperature in the second 0 oration is maintained at a ran e of 650 C to 750 C. being preferably at 25 C. at which temperature rapid roasting is effected and overheating of the char e avoided. Furthermore, during this second operation it is necessary to provide such conditions that a suflicient excess of air and a suflicient proportion of S0,is present in the furnace and that the proportion of carbon dioxide is maintained as low as possible.

In carrying out this second operation the ore after the preliminary roa'st' may be mixed with a proportion of material which has already been submitted to'a snl hating roast. For example I have found hat in treating a material in which the sulphur contents had been reduced to sulphide, four parts of the Same were mixed with one part of material which has already been roasted to sulphate whereby the said mixture contained approximately 4.8% sulphide sulphur and 2.2% sulphate sulphur. In this case a final product was obtained containing 5% sulphur present as sulphate.

The second alternative is adopted where the roasted calcihe is delivered to the reduction works with the sill hide sulphur reduced to a minimum and a so for the treat ment of oxidized ores.

In this case it is: necessary to add sulphide sulphur preferably the formcf green or unroasted concentrates during the sulphating operation. As the eflicienc'y ofthe sulphate formation increases as the ercentage' of sulphide sulphurdecreases it is preferable to add the green concentrates successively so re lating' the additions that the percentage '0 sulphide sulphur in the charge does not rise above 2%. p

, In the experiments whichlhavfibeen conducted b me the ores operated on "all contained a out 15%? 'of iron as oxideshndit would appear that this functions as a catalyst convertingthe sul bur dioxide in the presence of excess'of air to the' trioxide which is then absorbed by" the zinc'oxide to form the sulphate. I I y This ty e of roasting leads to the diminution of t e amount of feriitefoimed directly owingto the lower temperature, and indirectly owing "to the decompdsitionbf. the ferrite formed during the earlier stages ofthe roastby sulphuric anhydride formed during the'la'tter stages.

By these means it is possible to obtain such a degree of sulphating' as"fi 'ay be reuired for the purpose of introduclhg into t e leaching liquor sufficient sulphuric acid as sulphates to make up for losses'or for withdrawals from the solutions for other pur oses (such as the making of lithopone). urthermore a subsequent roasting or heating of 'calcines immediately precedent to the leaching may serve other useful urposes. In some cases especially where it is necessary to transport the roasted calci'ne from one place to another for treatment, it is customary to wet down thematerial to prevent dustingduring handling. In this and other ways chlorine is introduced which has been found prejudicialto the electro- 1 tic process and ithasb'een 'foundthat a nal roast as above described has the ifect of volatilizing the lchlorides and thereby eliminating'the same and at th'esame time convertin 'any remaining ferrous iron to ferric iron.

of air that the formation of a maximum amount of metallic sulphates is efiectcd.

2. A process for the roasting of zinc sulphide ores in which the sulphur contents are first reduced to from 6 to 10% (approximately) then diluting the sulphide sulphur contents by mixing the same with a proportion of ore relatively free from sulphide sulphur and then roasting at a relatively low temperature in the presence of an excess of air under such conditions that the formation of a maximum amount of metallic sulphates is effected.

3. A process for the roasting of zinc sulphide ores in which the sulphur contents are first reduced to from 6 to 10% approximately then diluting the sulphide sulphur contents by the addition of ore which has previously been subjected to a sulphating operation and then roasting the mixture at a relatively low temperature in the presence of an excess of air under such conditions that the formation of a maximum amount of metallic sulphates is effected.

4. In the roasting of zinc sulphide ores as hereinbefore described in which the sulphide sulphur contents are reduced to a relatively low percentage and then roasted at a relatively low temperature in presence of excess of air the addition oi a catalyst (such as ferric oxide) for the purpose of promoting the conversion of sulphur dioxide to sulphur trioxide for the formation of a maximum amount of metallic sulphates substantially as described.

In testimony whereof I have signed my name to this specification in the presence of two subscribing witnesses.

HERBERT W. GEPP.

Witnesses:

CLEM A. HAoK, Gr. CULLEN. 

